Enabling Ashcroft from within Ant is easy. You probably already have a target running a <junit> task somewhere in your build script. (If not you're in deep trouble). Ok, just amend it a little bit like this:

build.xml

It is imperative that you run JUnit in a forked JVM, otherwise the jvmarg will be ignored, and Ashcroft will not be activated. For maximum speed, we strongly recommend you also specify forkmode="once" (requires Ant 1.6.2 or higher).